1. Field of the Invention
This invention relates to an improved lithography system and method. More specifically, this invention relates to a lithography system and method using catadioptric exposure optics that projects high precision images without image flip.
2. Background Art
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays, circuit boards, various integrated circuits, and the like. A frequently used substrate for such applications is a semiconductor wafer. While this description is written in terms of a semiconductor wafer for illustrative purposes, one skilled in the art would recognize that this description also applies to other types of substrates known to those skilled in the art. During lithography, a wafer, which is disposed on a wafer stage, is exposed to an image projected onto the surface of the wafer by exposure optics located within a lithography apparatus. The image refers to the original, or source, image being exposed. The projected image refers to the image that actually contacts the surface of the wafer. While exposure optics are used in the case of photolithography, a different type of exposure apparatus may be used depending on the particular application. For example, x-ray or photon lithographies each may require a different exposure apparatus, as is known to those skilled in the art. The particular example of photolithography is discussed here for illustrative purposes only.
The projected image produces changes in the characteristics of a layer, for example photoresist, deposited on the surface of the wafer. These changes correspond to the features projected onto the wafer during exposure. Subsequent to exposure, the layer can be etched to produce a patterned layer. The pattern corresponds to those features projected onto the wafer during exposure. This patterned layer is then used to remove exposed portions of underlying structural layers within the wafer, such as conductive, semiconductive, or insulative layers. This process is then repeated, together with other steps, until the desired features have been formed on the surface of the wafer.
Step-and-scan technology works in conjunction with a projection optics system that has a narrow imaging slot. Rather than expose the entire wafer at one time, individual fields are scanned onto the wafer one at a time. This is done by moving the wafer and reticle simultaneously such that the imaging slot is moved across the field during the scan. The wafer stage must then be asynchronously stepped between field exposures to allow multiple copies of the reticle pattern to be exposed over the wafer surface. In this manner, the quality of the image projected onto the wafer is maximized.
Exposure optics comprise refractive and/or reflective elements, i.e., lenses and/or mirrors. Currently, most exposure optics used for commercial manufacturing consist only of lenses. However, the use of catadioptric (i.e., a combination of refractive and reflective elements) exposure optics is increasing. The use of refractive and reflective elements allow for a greater number of lithographic variables to be controlled during manufacturing. The use of mirrors, however, can lead to image flip problems.
There is a need for an optical system design that is capable of producing an unflipped (unmirrored) image on a semiconductor wafer. Furthermore, this optical system should be backward compatible to reticle designs in earlier catadioptric systems. Finally, there is a need for an optical system design capable of uniform and symmetric heat distribution in order to reduce a burden of thermal compensation system.
The present invention relates to a catadioptric lithographic exposure apparatus for forming an image pattern on a semiconductor wafer. Specifically, the present invention is a catadioptric lithographic system including a reticle optical group, a beam splitter, an aspheric mirror optical group, a folding mirror and a semiconductor wafer optical group. The light beam passes through an image pattern formed on the reticle onto the beam-splitter cube, where it is reflected onto the aspheric mirror. The aspheric mirror reflects the light beam back through the beam-splitter cube. A quarter wave plate, placed in an optical path between the aspheric mirror and the beam-splitter cube, changes polarization of the light entering the beam-splitter cube. After passing through the beams-splitter cube, the folding mirror, placed adjacent the beam-splitter cube, reflects the light beam. Another quarter wave plate, placed in an optical path between the fold mirror and the beam-splitter cube, changes polarization of the light entering the beam-splitter cube. Then, the light beam is reflected by the beam-splitter cube onto the wafer optical group. The wafer optical group magnifies, focuses and/or aligns the light beam, which subsequently forms a pattern on the semiconductor wafer. The pattern on the semiconductor wafer corresponds to the image pattern on the reticle.
In an embodiment of the present invention, a baffle plate is inserted between the beam splitter cube and the wafer optical group. The baffle plate serves to absorb a background scattered light generated as a result of internal reflections within the beam-splitter cube. The background scattered light is generated as a result of light beams passing through the beam-splitter cube and reflecting off beam splitter cube surfaces. Furthermore, the baffle plate substantially absorbs the background scattered light and does not reflect it back into the beam-splitter cube.
In another embodiment of the present invention, the beam-splitter cube includes two optical prism halves separated by a spacer plate. The width of the spacer plate is a variable in an offset created between the image axis of the object plane beam and the wafer plane beam. The object plane beam passes from the object plane through the reticle group of optics. The wafer plane beam passes through the wafer group. The width of the spacer plate depends on the amount of desired offset between the image axis of the object plane beam and the wafer plane beam. The width can be zero (in other words, optical surfaces of the two optical prism halves are in optical contact) in one embodiment or can vary in alternative embodiments. The width of the spacer plate determines the amount of background scattered light that is incident on the baffle plate and the wafer group. The spacer plate can be manufactured from a similar optical material as the optical prism halves of the beam-splitter cube. Furthermore, in an alternative embodiment, the beam-splitter cube includes two optical prism halves separated by a plurality of spacer plates. The plurality of spacer plates more effectively correct for bierefringence.
The systems and methods in the present invention provide a relatively uniform heat distribution. By allowing a double pass through the beam-splitter cube, the present invention symmetrically distributes heat generated by the passage of light through the system. This reduces the burden of the thermal compensation system.
Moreover, using the systems and methods in the present invention the light, passing from the reticle optical group, is folded an even number of times. This does not yield image mirroring and prevents improper flipped image formation on the semiconductor wafer.